There are a number of medical conditions which may be effectively treated with implantable medical devices. In many cases, the therapeutic effect of the implantable medical device may be enhanced, if the device includes a drug release coating including a therapeutic substance in a polymeric carrier. Two examples of implantable medical devices which may include such drug release coatings are endovascular stents and vascular filters.
Intravascular diseases are commonly treated by relatively non-invasive techniques such as percutaneous transluminal angioplasty (PTA) and percutaneous transluminal coronary angioplasty (PTCA). These angioplasty techniques typically involve the use of a guidewire and a balloon catheter. In these procedures, a balloon catheter is advanced over a guidewire such that the balloon is positioned proximate a restriction in a diseased vessel. The balloon is then inflated and the restriction in the vessel is opened.
While angioplasty techniques have gained wide acceptance, abrupt closure and restenosis have been identified as possible subsequent occurrences. Abrupt closure refers to the acute occlusion of a vessel immediately after or within the initial hours following a dilation procedure. Abrupt closure can result in myocardial infarction if blood flow is not restored in a timely manner. The primary mechanisms of abrupt closures are arterial dissection and/or thrombosis. Restenosis refers to the re-narrowing of an artery after an initially successful angioplasty. Restenosis occurs primarily within the initial six months after angioplasty, and is believed due to the proliferation and migration of the cellular components of the arterial wall.
Endovascular stents are placed in the dilated segment of a vessel lumen to mechanically block the effects of abrupt closure and restenosis. Such a stent is disclosed in U.S. Pat. No. 5,514,154 to Lau et al.
Recent developments have led to stents which can provide anti-thrombogenic and other medications to regions of a blood vessel which have been treated by angioplasty or to other interventional techniques. In U.S. Pat. No. 5,464,650, Berg et al. disclose a method for making an intravascular stent by applying to the stent, and in particular to its tissuecontacting surface, a solution which includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the solvent. After the solution is applied to the stent, the solvent is evaporated leaving the polymer/therapeutic agent surface treatment. Berg et. al. assert that these devices are capable of providing both short term medication delivery, over the initial hours and days after treatment, as well as long term medication delivery, over the weeks and months after treatment.
The process disclosed by Berg et al., which uses a polymeric carrier, is prone to the formation of polymeric surface imperfections during the coating process. This is especially evident on stents, which generally include many wire like members with interstitial spaces therebetween. The surface imperfections can include strands of drug laden polymeric material hanging loosely from or extending across the interstitial spaces in the stent. The imperfections can also include chunks or thickened coating portions at particular points relative to the rest of the coating. These imperfections, because of their drug delivering capabilities, may cause adverse effects. Loose strands or strands across interstitial spaces may not be secure, and thus, may enter the blood stream and fail to provide local treatment. If these agents are released to locations other than the targeted area, unwanted side effects may result. An uneven coating may also result in non-uniform treatment of the vessel wall.
As mentioned previously, vascular filters are another type of implantable medical device which may benefit from the inclusion of a drug release coating including a therapeutic substance in a polymeric carrier. One of the most common applications for vascular filters is the treatment of Deep Venous Thrombosis (DVT). Deep Venous Thrombosis patients experience clotting of blood in the large veins of the lower portions of the body. These patients are constantly at risk of a clot breaking free and traveling via the inferior vena cava to the heart and lungs. This process is known as pulmonary embolization. Puhnonary embolization can frequently be fatal, because large blood clots can interfere with the function of the heart and/or lungs. For example, a large blood clot which becomes trapped proximate the heart may interfere with the life-sustaining pumping action of the heart. A blood clot which passes through the heart will be pumped into the lungs and may cause a blockage in the pulmonary arteries. A blockage of this type in the lungs will interfere with the oxygenation of the blood causing shock or death.
Pulmonary embolization may be successfully prevented by the appropriate placement of a vascular filter in the vascular system of a patient's body. Placement of the filter may be accomplished by performing a laparotomy with the patient under general anesthesia. However, intravenous insertion is often the preferred method of placing a vascular in a patient's vascular system.
In the treatment of Deep Venous Thrombosis, a vascular filter is placed in the inferior vena cava of a patient. The inferior vena cava is a large vessel which returns blood to the heart from the lower part of the body. The inferior vena cava may be accessed through the patient's femoral vein.
Vascular filters may be placed in other locations when treating other conditions. For example, if blood clots are expected to approach the heart and lungs from the upper portion of the body, a vascular filter may be positioned in the superior vena cava. The superior vena cava is a large vessel which returns blood to the heart from the upper part of the body. The superior vena cava may by accessed through the jugular vein, located in the patient's neck. Once placed inside a blood vessel, a vascular filter acts to catch and hold blood clots. The flow of blood around the captured clots allows the body's lysing process to dissolve the clots.